Design and Analysisof Composite Structures

Christos Kassapoglou

Second Edition

Aerospace SeriesEditors Peter Belobaba, Jonathan Cooper,

Roy Langton and Allan Seabridge

With Applications to Aerospace Structures

DESIGN AND ANALYSISOF COMPOSITESTRUCTURES

Aerospace Series List

Design and Analysis of Composite Structures: WithApplications to Aerospace Structures, Second Edition

Kassapoglou April 2013

Aircraft Systems Integration of Air-Launched Weapons Rigby April 2013

Design and Development of Aircraft Systems, Second Edition Moir and Seabridge November 2012

Understanding Aerodynamics: Arguing from the Real Physics McLean November 2012

Aircraft Design: A Systems Engineering Approach Sadraey October 2012

Introduction to UAV Systems, Fourth Edition Fahlstrom andGleason

August 2012

Theory of Lift: Introductory Computational Aerodynamicswith MATLAB and Octave

McBain August 2012

Sense and Avoid in UAS: Research and Applications Angelov April 2012

Morphing Aerospace Vehicles and Structures Valasek April 2012

Gas Turbine Propulsion Systems MacIsaac andLangton

July 2011

Basic Helicopter Aerodynamics, Third Edition Seddon andNewman

July 2011

Advanced Control of Aircraft, Spacecraft and Rockets Tewari July 2011

Cooperative Path Planning of Unmanned Aerial Vehicles Tsourdos et al November 2010

Principles of Flight for Pilots Swatton October 2010

Air Travel and Health: A Systems Perspective Seabridge et al September 2010

Unmanned Aircraft Systems: UAVS Design, Developmentand Deployment

Austin April 2010

Introduction to Antenna Placement and Installations Macnamara April 2010

Principles of Flight Simulation Allerton October 2009

Aircraft Fuel Systems Langton et al May 2009

The Global Airline Industry Belobaba April 2009

Computational Modelling and Simulation of Aircraft andthe Environment: Volume 1 - Platform Kinematics andSynthetic Environment

Diston April 2009

Handbook of Space Technology Ley, WittmannHallmann

April 2009

Aircraft Performance Theory and Practice for Pilots Swatton August 2008

Aircraft Systems, Third Edition Moir and Seabridge March 2008

Introduction to Aircraft Aeroelasticity and Loads Wright and Cooper December 2007

Stability and Control of Aircraft Systems Langton September 2006

Military Avionics Systems Moir and Seabridge February 2006

Design and Development of Aircraft Systems Moir and Seabridge June 2004

Aircraft Loading and Structural Layout Howe May 2004

Aircraft Display Systems Jukes December 2003

Civil Avionics Systems Moir and Seabridge December 2002

DESIGN AND ANALYSISOF COMPOSITESTRUCTURESWITH APPLICATIONS TOAEROSPACE STRUCTURES

Second Edition

Christos KassapoglouDelft University of Technology, The Netherlands

A John Wiley & Sons, Ltd., Publication

This edition first published 2013C© 2013, John Wiley & Sons Ltd

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Library of Congress Cataloging-in-Publication Data

A catalogue record for this book is available from the British Library.

ISBN: 9781118401606

Typeset in 10/12pt Times by Aptara Inc., New Delhi, India

Contents

About the Author xi

Series Preface xiii

Preface to First Edition xv

Preface to Second Edition xix

1 Applications of Advanced Composites in Aircraft Structures 1References 7

2 Cost of Composites: a Qualitative Discussion 92.1 Recurring Cost 102.2 Nonrecurring Cost 182.3 Technology Selection 202.4 Summary and Conclusions 27

Exercises 30References 31

3 Review of Classical Laminated Plate Theory 333.1 Composite Materials: Definitions, Symbols and Terminology 333.2 Constitutive Equations in Three Dimensions 35

3.2.1 Tensor Transformations 383.3 Constitutive Equations in Two Dimensions: Plane Stress 40

Exercises 52References 53

4 Review of Laminate Strength and Failure Criteria 554.1 Maximum Stress Failure Theory 574.2 Maximum Strain Failure Theory 584.3 Tsai–Hill Failure Theory 584.4 Tsai–Wu Failure Theory 594.5 Puck Failure Theory 594.6 Other Failure Theories 61

References 62

vi Contents

5 Composite Structural Components and Mathematical Formulation 655.1 Overview of Composite Airframe 65

5.1.1 The Structural Design Process: The Analyst’s Perspective 665.1.2 Basic Design Concept and Process/Material Considerations

for Aircraft Parts 715.1.3 Sources of Uncertainty: Applied Loads, Usage and Material Scatter 74

5.1.3.1 Knowledge of Applied Loads 755.1.3.2 Variability in Usage 755.1.3.3 Material Scatter 75

5.1.4 Environmental Effects 775.1.5 Effect of Damage 785.1.6 Design Values and Allowables 805.1.7 Additional Considerations of the Design Process 83

5.2 Governing Equations 845.2.1 Equilibrium Equations 845.2.2 Stress–Strain Equations 865.2.3 Strain–Displacement Equations 875.2.4 von Karman Anisotropic Plate Equations for Large Deflections 88

5.3 Reductions of Governing Equations: Applicationsto Specific Problems 945.3.1 Composite Plate under Localized In-Plane Load 945.3.2 Composite Plate under Out-of-Plane Point Load 105

5.4 Energy Methods 1085.4.1 Energy Expressions for Composite Plates 109

5.4.1.1 Internal Strain Energy U 1105.4.1.2 External Work W 113

Exercises 115References 122

6 Buckling of Composite Plates 1256.1 Buckling of Rectangular Composite Plate under Biaxial Loading 1256.2 Buckling of Rectangular Composite Plate under Uniaxial Compression 129

6.2.1 Uniaxial Compression, Three Sides Simply Supported,One Side Free 131

6.3 Buckling of Rectangular Composite Plate under Shear 1336.4 Buckling of Long Rectangular Composite Plates under Shear 1366.5 Buckling of Rectangular Composite Plates under Combined Loads 1386.6 Design Equations for Different Boundary Conditions and Load Combinations 145

Exercises 145References 152

7 Post-Buckling 1537.1 Post-Buckling Analysis of Composite Panels under Compression 157

7.1.1 Application: Post-Buckled Panel under Compression 165

Contents vii

7.2 Post-Buckling Analysis of Composite Plates under Shear 1687.2.1 Post-Buckling of Stiffened Composite Panels under Shear 172

7.2.1.1 Application: Post-Buckled Stiffened Fuselage Skinunder Shear 177

7.2.2 Post-Buckling of Stiffened Composite Panels under CombinedUniaxial and Shear Loading 180

Exercises 181References 187

8 Design and Analysis of Composite Beams 1898.1 Cross-Section Definition Based on Design Guidelines 1898.2 Cross-Sectional Properties 1938.3 Column Buckling 1998.4 Beam on an Elastic Foundation under Compression 2008.5 Crippling 205

8.5.1 One-Edge-Free (OEF) Crippling 2078.5.2 No-Edge-Free (NEF) Crippling 2118.5.3 Crippling under Bending Loads 214

8.5.3.1 Application: Stiffener Design under Bending Loads 2158.5.4 Crippling of Closed-Section Beams 219

8.6 Importance of Radius Regions at Flange Intersections 2198.7 Inter-Rivet Buckling of Stiffener Flanges 2228.8 Application: Analysis of Stiffeners in a Stiffened Panel under Compression 227

Exercises 230References 235

9 Skin–Stiffened Structure 2379.1 Smearing of Stiffness Properties (Equivalent Stiffness) 237

9.1.1 Equivalent Membrane Stiffnesses 2379.1.2 Equivalent Bending Stiffnesses 239

9.2 Failure Modes of a Stiffened Panel 2419.2.1 Local Buckling (between Stiffeners) versus Overall Panel Buckling

(the Panel Breaker Condition) 2429.2.1.1 Global Buckling = Local Buckling (Compression

Loading) 2439.2.1.2 Stiffener Buckling = PB × Buckling of Skin between

Stiffeners (Compression Loading) 2469.2.1.3 Example 249

9.2.2 Skin–Stiffener Separation 2509.3 Additional Considerations for Stiffened Panels 265

9.3.1 ‘Pinching’ of Skin 2659.3.2 Co-curing versus Bonding versus Fastening 266Exercises 267References 272

viii Contents

10 Sandwich Structure 27510.1 Sandwich Bending Stiffnesses 27610.2 Buckling of Sandwich Structure 278

10.2.1 Buckling of Sandwich under Compression 27810.2.2 Buckling of Sandwich under Shear 28010.2.3 Buckling of Sandwich under Combined Loading 281

10.3 Sandwich Wrinkling 28110.3.1 Sandwich Wrinkling under Compression 28210.3.2 Sandwich Wrinkling under Shear 29310.3.3 Sandwich Wrinkling under Combined Loads 293

10.4 Sandwich Crimping 29510.4.1 Sandwich Crimping under Compression 29510.4.2 Sandwich Crimping under Shear 295

10.5 Sandwich Intracellular Buckling (Dimpling) under Compression 29610.6 Attaching Sandwich Structures 296

10.6.1 Core Ramp-Down Regions 29710.6.2 Alternatives to Core Ramp-Down 299Exercises 301References 306

11 Composite Fittings 30911.1 Challenges in Creating Cost- and Weight-Efficient Composite Fittings 30911.2 Basic Fittings 311

11.2.1 Clips 31111.2.1.1 Tension Clips 31111.2.1.2 Shear Clips 322

11.2.2 Lugs 32811.2.2.1 Lug under Axial Loads 32811.2.2.2 Lug under Transverse Loads 33311.2.2.3 Lug under Oblique (Combined) Loads 337

11.3 Other Fittings 33911.3.1 Bathtub Fittings 33911.3.2 Root Fittings 340Exercises 340References 341

12 Good Design Practices and Design ‘Rules of Thumb’ 34312.1 Layup/Stacking Sequence-related 34312.2 Loading and Performance-related 34412.3 Guidelines Related to Environmental Sensitivity and Manufacturing

Constraints 34512.4 Configuration and Layout-related 347

Exercises 348References 349

Contents ix

13 Application – Design of a Composite Panel 35113.1 Monolithic Laminate 35113.2 Stiffened Panel Design 36213.3 Sandwich Design 37313.4 Cost Considerations 38113.5 Comparison and Discussion 382

References 385

Index 387

About the Author

Christos Kassapoglou received his BS degree in Aeronautics and Astronautics, and twoMS degrees (Aeronautics and Astronautics and Mechanical Engineering) all from theMassachusetts Institute of Technology. He received his Ph.D in Aerospace Engineering fromthe Delft University of Technology. Since 1984, he has worked in the industry, first at BeechAircraft on the all-composite Starship I and then at Sikorsky Aircraft in the Structures ResearchGroup specializing on analysis of composite structures of the all-composite Comanche andother helicopters, and leading internally funded research and programs funded by NASA andthe US Army. Since 2001, he has been consulting with various companies in the United Stateson applications of composite structures on airplanes and helicopters. He joined the facultyof the Aerospace Engineering Department of the Delft University of Technology (AerospaceStructures) in 2007 as an Associate Professor. His interests include fatigue and damage tol-erance of composites, analysis of sandwich structures, design and optimization for cost andweight and technology optimization. He has over 40 journal papers and three issued or pendingpatents on related subjects. He is a member of AIAA, AHS and SAMPE.

Series Preface

The field of aerospace is wide ranging and covers a variety of products, disciplines, anddomains, not merely in engineering but in many related supporting activities. These combineto enable the aerospace industry to produce exciting and technologically challenging products.A wealth of knowledge is contained by practitioners and professionals in the aerospace fields,which is of benefit to other practitioners in the industry, and to those entering the industryfrom University.

The Aerospace Series aims to be a practical and topical series of books aimed at engineeringprofessionals, operators, users and allied professions such as commercial and legal executivesin the aerospace industry. The range of topics is intended to be wide ranging, covering designand development, manufacture, operation and support of aircraft, as well as topics such asinfrastructure operations and developments in research and technology. The intention is toprovide a source of relevant information that will be of interest and benefit to all those peopleworking in aerospace.

The use of composite materials for aerospace structures has increased dramatically in thelast three decades. The attractive strength-to-weight ratios, improved fatigue and corrosionresistance and ability to tailor the geometry and fibre orientations, combined with recentadvances in fabrication, have made composites a very attractive option for aerospace appli-cations from both a technical and financial viewpoint. This has been tempered by problemsassociated with damage tolerance and detection, damage repair, environmental degradationand assembly joints. The anisotropic nature of composites also dramatically increases thenumber of variables that need to be considered in the design of any aerospace structure.

This book, Design and Analysis of Composite Structures: With Application to AerospaceStructures, Second Edition, provides a methodology of various analysis approaches that canbe used for the preliminary design of aerospace structures without having to resort to finiteelements. Representative types of composite structure are described, along with techniques todefine the geometry and lay-up stacking sequence required to withstand the applied loads. Thevalue of such a set of tools is to enable rapid initial trade-off preliminary design studies to bemade, before using a detailed finite element analysis on the finalized design configurations.

Allan SeabridgeJonathan Cooper

Peter Belobaba

Preface to First Edition

This book is a compilation of analysis and design methods for structural components madeof advanced composites. The term “advanced composites” is used here somewhat looselyand refers to materials consisting of a high-performance fibre (graphite, glass, Kevlar R©, etc.)embedded in a polymeric matrix (epoxy, bismaleimide, PEEK, etc.). The material in this bookis the product of lecture notes used in graduate-level classes in Advanced Composites Designand Optimization courses taught at the Delft University of Technology.

The book is aimed at fourth year undergraduate or graduate-level students and startingengineering professionals in the composites industry. The reader is expected to be familiarwith classical laminated-plate theory (CLPT) and first ply failure criteria. Also, some awarenessof energy methods and Rayleigh–Ritz approaches will make some of the solution methodseasier to follow. In addition, basic applied mathematics knowledge such as Fourier series,simple solutions of partial differential equations and calculus of variations are subjects thatthe reader should have some familiarity with.

A series of attractive properties of composites such as high stiffness and strength-to-weightratios, reduced sensitivity to cyclic loads, improved corrosion resistance and, above all, theability to tailor the configuration (geometry and stacking sequence) to specific loading condi-tions for optimum performance has made them a prime candidate material for use in aerospaceapplications. In addition, the advent of automated fabrication methods such as advancedfibre/tow placement, automated tape laying, filament winding, has made it possible to producecomplex components at costs competitive with if not lower than metallic counterparts. Thisincrease in the use of composites has brought to the forefront the need for reliable analysis anddesign methods that can assist engineers in implementing composites in aerospace structures.This book is a small contribution towards fulfilling that need.

The objective is to provide methodology and analysis approaches that can be used inpreliminary design. The emphasis is on methods that do not use finite elements or othercomputationally expensive approaches in order to allow the rapid generation of alternativedesigns that can be traded against each other. This will provide insight in how different designvariables and parameters of a problem affect the result.

The approach to preliminary design and analysis may differ according to the application andthe persons involved. It combines a series of attributes such as experience, intuition, inspirationand thorough knowledge of the basics. Of these, intuition and inspiration cannot be capturedin the pages of a book or itemized in a series of steps. For the first attribute, experience, anattempt can be made to collect previous best practices which can serve as guidelines for futurework. Only the last attribute, knowledge of the basics, can be formulated in such a way that the

xvi Preface to First Edition

reader can learn and understand them and then apply them to his/her own applications. Anddoing that is neither easy nor guaranteed to be exhaustive. The wide variety of applications andthe peculiarities that each may require in the approach, preclude any complete and in-depthpresentation of the material. It is only hoped that the material presented here will serve as astarting point for most types of design and analysis problems.

Given these difficulties, the material covered in this book is an attempt to show representativetypes of composite structure and some of the approaches that may be used in determiningthe geometry and stacking sequences that meet applied loads without failure. It should beemphasized that not all methods presented here are equally accurate nor do they have thesame range of applicability. Every effort has been made to present, along with each approach,its limitations. There are many more methods than the ones presented here and they varyin accuracy and range of applicability. Additional references are given where some of thesemethods can be found.

These methods cannot replace thorough finite element analyses which, when properly setup, will be more accurate than most of the methods presented here. Unfortunately, the com-plexity of some of the problems and the current (and foreseeable) computational efficiencyin implementing finite element solutions precludes their extensive use during preliminarydesign or, even, early phases of the detailed design. There is not enough time to tradehundreds or thousands of designs in an optimization effort to determine the “best” designif the analysis method is based on detailed finite elements. On the other hand, once thedesign configuration has been finalized or a couple of configurations have been downselectedusing simpler, more efficient approaches, detailed finite elements can and should be usedto provide accurate predictions for the performance, point to areas where revisions of thedesign are necessary, and, eventually, provide supporting analysis for the certification effort ofa product.

Some highlights of composite applications from the 1950s to today are given in Chapter 1with emphasis on nonmilitary applications. Recurring and nonrecurring cost issues that mayaffect design decisions are presented in Chapter 2 for specific fabrication processes. Chapter 3provides a review of CLPT and Chapter 4 summarizes strength failure criteria for compositeplates; these two chapters are meant as a quick refresher of some of the basic concepts andequations that will be used in subsequent chapters.

Chapter 5 presents the governing equations for anisotropic plates. It includes the vonKarman large deflection equations that are used later to generate simple solutions for post-buckled composite plates under compression. These are followed by a presentation of the typesof composite parts found in aerospace structures and the design philosophy typically used tocome up with a geometric shape. Design requirements and desired attributes are also discussed.This sets the stage for quantitative requirements that address uncertainties during the designand during service of a fielded structure. Uncertainties in applied loads and variations in usagefrom one user to another are briefly discussed. A more detailed discussion about uncertaintiesin material performance (material scatter) leads to the introduction of statistically meaningful(A- and B-basis) design values or allowables. Finally, sensitivity to damage and environ-mental conditions is discussed and the use of knockdown factors for preliminary design isintroduced.

Chapter 6 contains a discussion of buckling of composite plates. Plates are introducedfirst and beams follow (Chapter 8) because failure modes of beams such as crippling can

Preface to First Edition xvii

be introduced more easily as special cases of plate buckling and post-buckling. Bucklingunder compression is discussed first, followed by buckling under shear. Combined loadcases are treated next and a table including different boundary conditions and load cases isprovided.

Post-buckling under compression and shear is treated in Chapter 7. For applied compression,an approximate solution to the governing (von Karman) equations for large deflections of platesis presented. For applied shear, an approach that is a modification of the standard approachfor metals undergoing diagonal tension is presented. A brief section follows suggesting howpost-buckling under combined compression and shear could be treated.

Design and analysis of composite beams (stiffeners, stringers, panel breakers, etc.) aretreated in Chapter 8. Calculation of equivalent membrane and bending stiffnesses for crosssections consisting of members with different layups are presented first. These can be usedwith standard beam design equations and some examples are given. Buckling of beams andbeams on elastic foundations is discussed next. This does not differentiate between metalsand composites. The standard equations for metals can be used with appropriate (re)definitionof terms such as membrane and bending stiffness. The effect of different end conditions isalso discussed. Crippling, or collapse after very-short-wavelength buckling, is discussed indetail deriving design equations from plate buckling presented earlier and from semi-empiricalapproaches. Finally, conditions for inter-rivet buckling are presented.

The two constituents, plates and beams are brought together in Chapter 9 where stiffenedpanels are discussed. The concept of smeared stiffness is introduced and its applicability isdiscussed briefly. Then, special design conditions such as the panel breaker condition andfailure modes such as skin–stiffener separation are analysed in detail, concluding with designguidelines for stiffened panels derived from the previous analyses.

Sandwich structure is treated in Chapter 10. Aspects of sandwich modelling, in particular,the effect of transverse shear on buckling, are treated first. Various failure modes such aswrinkling, crimping and intracellular buckling are then discussed with particular emphasison wrinkling with and without waviness. Interaction equations are introduced for analysingsandwich structure under combined loading. A brief discussion on attachments including rampdowns and associated design guidelines close this chapter.

The final chapter, Chapter 11, summarizes design guidelines and rules presented throughoutthe previous chapters. It also includes some additional rules, presented for the first time in thisbook, that have been found to be useful in designing composite structures.

To facilitate material coverage and in order to avoid having to read some chapters that maybe considered of lesser interest or not directly related to the reader’s needs, certain conceptsand equations are presented in more than one place. This is minimized to avoid repetition andis done in such a way that reader does not have to interrupt reading a certain chapter and goback to find the original concept or equation on which the current derivation is based.

Specific problems are worked out in detail as examples of applications throughout the book.Representative exercises are given at the end of each chapter. These require the determinationof geometry and/or stacking sequence for a specific structure not to fail under certain appliedloads. Many of them are created in such a way that more than one answer is acceptable reflectingreal-life situations. Depending on the assumptions made and design rules enforced, differentbut still acceptable designs can be created. Even though low weight is the primary objectiveof most of the exercises, situations where other issues are important and end up driving the

xviii Preface to First Edition

design are also given. For academic applications, experience has shown that students benefitthe most if they work out some of these exercises in teams, so design ideas and concepts can bediscussed and an approach to a solution formulated. It is recognized that analysis of compositestructures is very much in a state of flux, and new and better methods are being developed(e.g. failure theories with and without damage). The present edition includes what are felt tobe the most useful approaches at this point in time. As better approaches mature in the future,it will be modified accordingly.

Preface to Second Edition

The first edition of this book met with sufficient interest to justify an improved and enhancedsecond edition. Feedback from the readers of the first edition led to the addition of two newchapters, one on fittings and one with an example design problem of larger structural parts.

The objective of this book is to introduce basic design and analysis methods of compositestructures with sufficient theoretical background for the reader to be able to (a) develop his/herown methods for other situations and (b) extend the present methods to improve their accuracyand applicability. Balancing this objective with the need to provide simple design equationsis difficult. Invariably, in some areas the theoretical background will appear too extensivewhile, in other cases, equations are presented without sufficient derivation. In addition, peoplewho have worked on any of the topics in any detail will, probably, complain that some of theapproaches are oversimplified.

Trying to satisfy all the anticipated readers in all aspects covered in this book is simplyimpossible. If I can get about the same number of complaints at the two extremes of toodetailed analysis and oversimplified analysis, I will have achieved my goal. After all, I onlyhope to give some guidelines for approaches I found useful that the reader can build on andapply to his/her own situations.

The changes from the first edition are (a) addition of more exercises in most chapters, (b)addition of the Puck failure criterion in Chapter 4, (c) addition of a chapter (Chapter 11) onanalysis of simple fittings and (d) addition of a chapter (Chapter 13) with a detailed case studyfor designing a fuselage panel with three different design concepts. Furthermore, a diligentattempt was also made to correct typos of the first edition.

Perhaps, the most important and useful feature of this second edition is that it is accompaniedby an App. The App is called CoDeAn (Composites Design and Analysis). Most equationsand design procedures in the book have been incorporated in CoDeAn in a user-friendly andefficient manner. This allows iPhone and iPad users to perform analyses and design studiesat the touch of a button. A material library with easy addition and modification options isavailable. Classical laminated-plate theory, first ply failure and buckling analysis are alreadyavailable with this release. Future releases will include the rest of the book.

1Applications of AdvancedComposites in Aircraft Structures

Some of the milestones in the implementation of advanced composites on aircraft and rotorcraftare discussed in this chapter. Specific applications have been selected that highlight variousphases that the composites industry went through while trying to extend the application ofcomposites.

The application of composites in civilian or military aircraft followed the typical stagesthat every new technology goes through during its implementation. At the beginning, limitedapplication on secondary structure minimized risk and improved understanding by collectingdata from tests and fleet experience. This limited usage was followed by wider applications,first in smaller aircraft, capitalizing on the experience gained earlier. More recently, with theincreased demand on efficiency and low operation costs, composites are being applied widelyon larger aircraft.

Perhaps the first significant application of advanced composites was on the Akaflieg PhonixFS-24 (Figure 1.1) in the late 1950s. What started as a balsa wood and paper sailplane designedby professors at the University of Stuttgart and built by the students was later transformed intoa fibreglass/balsa wood sandwich design. Eight planes were eventually built.

The helicopter industry was among the first to recognize the potential of the composite mate-rials and use them on primary structure. The main and tail rotor blades with their beam-likebehaviour were one of the major structural parts designed and built with composites towardsthe end of the 1960s. One such example is the Aerospatiale Gazelle (Figure 1.2). Even though,to first order, helicopter blades can be modelled as beams, the loading complexity and the mul-tiple static and dynamic performance requirements (strength, buckling, stiffness distribution,frequency placement, etc.) make for a very challenging design and manufacturing problem.

In the 1970s, with the composites usage on sailplanes and helicopters increasing, the first all-composite planes appeared. These were small recreational or aerobatic planes. Most notableamong them were the Burt Rutan designs such as the Long EZ and Vari-Eze (Figure 1.3).These were largely co-cured and bonded constructions with very limited numbers of fasteners.Efficient aerodynamic designs with mostly laminar flow and light weight led to a combinationof speed and agility.

Design and Analysis of Composite Structures: With Applications to Aerospace Structures, Second Edition. Christos Kassapoglou.© 2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd.

2 Design and Analysis of Composite Structures

Figure 1.1 Akaflieg Phonix FS-24 (Courtesy of Deutsches Segelflugzeugmuseum; see Plate 1 for thecolour figure)

Figure 1.2 Aerospatiale SA 341G Gazelle (Copyright Jenny Coffey, printed with permission; seePlate 2 for the colour figure)

Figure 1.3 Long EZ and Vari-Eze. (Vari-Eze photo: courtesy of Stephen Kearney; Long EZ photo:courtesy of Ray McCrea; see Plate 3 for the colour figure)

Up to that point, usage of composites was limited and/or was applied to small aircraftwith relatively easy structural requirements. In addition, the performance of composites wasnot completely understood. For example, their sensitivity to impact damage and its implica-tions for design only came to the forefront in the late 1970s and early 1980s. At that time,efforts to build the first all-composite airplane of larger size began with the LearFan 2100

Applications of Advanced Composites in Aircraft Structures 3

Figure 1.4 LearAvia LearFan 2100 (Copyright Thierry Deutsch; see Plate 4 for the colour figure)

(Figure 1.4). This was the first civil aviation all-composite airplane to seek FAA certification(see Section 2.2). It used a pusher propeller and combined high speed and low weight withexcellent range and fuel consumption. Unfortunately, while it met all the structural certificationrequirements, delays in certifying the drive system and the death of Bill Lear the visionarydesigner and inventor behind the project, kept the LearFan from making it into production andthe company, LearAvia, went bankrupt.

The Beech Starship I (Figure 1.5) which followed on the heels of the LearFan in the early1980s was the first all-composite airplane to obtain FAA certification. It was designed to thenew composite structure requirements specially created for it by the FAA. These requirementswere the precursor of the structural requirements for composite aircraft as they are today. Unlikethe LearFan which was a more conventional skin-stiffened structure with frames and stringers,the Starship fuselage was made of sandwich (graphite/epoxy facesheets with Nomex R© core)and had a very limited number of frames, increasing cabin head room for a given cabindiameter and minimizing fabrication cost. It was co-cured in large pieces that were bondedtogether and, in critical connections such as the wing-box or the main fuselage joints, werealso fastened. Designed also by Burt Rutan, the Starship was meant to have mostly laminar

Figure 1.5 Beech (Raytheon Aircraft) Starship I (Courtesy of Brian Bartlett; see Plate 5 for the colourfigure)

4 Design and Analysis of Composite Structures

Figure 1.6 Airbus A-320 (Courtesy of Brian Bartlett; see Plate 6 for the colour figure)

flow and increased range through the use of efficient canard design and blended main wing.Two engines with pusher propellers located at the aft fuselage were to provide enough powerfor high cruising speed. In the end, the aerodynamic performance was not met and the fuelconsumption and cruising speeds missed their targets by a small amount. However, structurallythe Starship I proved that the all-composite aircraft could be designed and fabricated to meetthe stringent FAA requirements. In addition, invaluable experience was gained in analysisand testing of large composite structures and new low-cost structurally robust concepts weredeveloped for joints and sandwich structure in general.

With fuel prices rising, composites with their reduced weight became a very attractivealternative to the metal structure. Applications in the large civilian transport category startedin the early 1980s with the Boeing 737 horizontal stabilizer which was a sandwich constructionand continued with larger-scale application on the Airbus A-320 (Figure 1.6). The horizontaland vertical stabilizers as well as the control surfaces of the A-320 are made of compositematerials.

The next significant application of composites on primary aircraft structure came in the1990s with the Boeing 777 (Figure 1.7) where, in addition to the empennage and controlsurfaces, the main floor beams are also made out of composites.

Figure 1.7 Boeing 777 (Courtesy of Brian Bartlett; see Plate 7 for the colour figure)

Applications of Advanced Composites in Aircraft Structures 5

Figure 1.8 Airbus A-380 (Courtesy of Bjoern Schmitt, World of Aviation.de; see Plate 8 for the colourfigure)

Despite the use of innovative manufacturing technologies which started with early roboticsapplications on the A320 and continued with significant automation (tape layup) on the 777,the cost of composite structures was not attractive enough to lead to an even larger-scale(e.g. entire fuselage and/or wing structure) application of composites at that time. The AirbusA-380 (Figure 1.8) in the new millennium, was the next major application with glass/aluminium(glare) composites on the upper portion of the fuselage and glass and graphite composites inthe centre wing-box, floor beams and aft pressure bulkhead.

Already in the 1990s, the demand for more efficient aircraft with lower operation andmaintenance costs made it clear that more usage of composites was necessary for significantreductions in weight in order to gain in fuel efficiency. In addition, improved fatigue lives andimproved corrosion resistance compared with aluminium suggested that more composites onaircraft were necessary. This, despite the fact that the cost of composites was still not com-petitive with aluminium and the stringent certification requirements would lead to increasedcertification cost.

Boeing was the first to commit to a composite fuselage and wing with the 787 (Figure 1.9)launched in the first decade of the new millennium. Such extended use of composites, about50% of the structure (combined with other advanced technologies) would give the efficiencyimprovement (increased range, reduced operation and maintenance costs) needed by the airlineoperators.

Figure 1.9 Boeing 787 Dreamliner (Courtesy of Agnes Blom; see Plate 9 for the colour figure)

6 Design and Analysis of Composite Structures

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Figure 1.10 Applications of composites in military and civilian aircraft structures

The large number of orders (most successful launch in history) for the Boeing 787 ledAirbus to start development of a competing design in the market segment covered by the 787and the 777. This is the Airbus A-350, with all-composite fuselage and wings.

Another way to see the implementation of composites in aircraft structure over time is byexamining the amount of composites (by weight) used in various aircraft models as a functionof time. This is shown in Figure 1.10 for some civilian and military aircraft. It should be bornein mind that the numbers shown in Figure 1.10 are approximate as they had to be inferredfrom open literature data and interpretation of different company announcements [1–8].

Both military and civilian aircraft applications show the same basic trends. A slow start(corresponding to the period where the behaviour of composite structures is still not wellunderstood and limited low risk applications are selected) is followed by rapid growth asexperience is gained, reliable analysis and design tools are developed and verified by testingand the need for reduced weight becomes more pressing. After the rapid growth period, theapplicability levels off as: (a) it becomes harder to find parts of the structure that are amenableto the use of composites; (b) the cost of further composite implementation becomes prohibitive;and (c) managerial decisions and other external factors (lack of funding, changes in researchemphasis, investments already made in other technologies) favour alternatives. As might beexpected, composite implementation in military aircraft leads the way. The fact that in recentyears civilian applications seem to have overtaken military applications does not reflect truetrends as much as lack of data on the military side (e.g. several military programs such as theB-2 have very large composite applications, but the actual numbers are hard to find).

It is still unclear how well the composite primary structures in the most recent programs suchas the Boeing 787 and the Airbus A-350 will perform and whether they will meet the design

Applications of Advanced Composites in Aircraft Structures 7

targets. In addition, several areas such as the performance of composites after impact, fatigueand damage tolerance are still the subjects of ongoing research. As our understanding in theseareas improves, the development cost, which currently requires a large amount of testing toanswer questions where analysis is prohibitively expensive and/or not as accurate as neededto reduce the amount of testing, will drop significantly. In addition, further improvements inrobotics technology and integration of parts into larger co-cured structures are expected tomake the fabrication cost of composites more competitive compared with metal airplanes.

References

[1] Jane’s All the World’s Aircraft 2000–2001, P. Jackson (ed.), Jane’s Information Group, 2000.[2] NetComposites News, 14 December 2000.[3] Aerospace America, May 2001, 45–47.[4] Aerospace America, September 2005.[5] www.compositesworld.com/articles/boeing-sets-pace-for-composite-usage-in-large-civil-aircraft.[6] www.boeing.com/commercial/products.html.[7] Deo, R.B., Starnes, J.H., Holzwarth, R.C. Low-Cost Composite Materials and Structures for Aircraft Applications.

Presented at the RTO AVT Specialists’ Meeting on Low Cost Composite Structures; May 7-11, 2001 and publishedin RTO-MP-069(II); Loen, Norway.

[8] Watson, J.C. and Ostrodka, D.L., AV-8B Forward Fuselage Development. Proceedings of 5th Conference onFibrous Composites in Structural Design; 1981 January; New Orleans, LA.